High Crimes

Brown-headed Cowbird enters an oriole nest

Curious, I thought.

A female grosbeak went inside a Baltimore Oriole nest. At least, that’s what I thought I saw. But after I posted a video on Facebook and Twitter, several friends corrected my error. The interloper was a Brown-headed Cowbird.

And that’s not good news for the orioles.

It was day 10 of the orioles’ construction project. The nest had become a deep pouch, and Lady Baltimore was working on the interior’s finishing touches. Unable to predict the timing of her comings and goings, I was (and am) in the habit of focusing the camera on the nest, hitting ‘record’ and stepping away. So, I noticed the interloper’s visit only when I reviewed the video later that day—to my regret. Had I seen the bird live, I might have adjusted the camera’s focus for a clearer image. At least that’s my excuse for this case of mistaken avian identity.

Here’s what I saw:

Here’s a photo of a female cowbird (not mine; taken by Kurt Bauschardt, and used here under a Creative Commons Attribution license).

Female Brown-headed Cowbird perched on a branch
Female Brown-headed Cowbird

And, here’s my photo of a female Rose-breasted Grosbeak.

Female Rose-breasted Grosbeak perches on a branch.
Female Rose-breasted Grosbeak

Ok… about all they have in common is that wide beak. So much for my bird ID skills.

So, what’s the issue when a cowbird visits an oriole nest? Cowbirds are brood parasites, laying their eggs in other birds’ nests. They don’t build nests. They don’t incubate eggs or raise their offspring from hatchling to nestling to fledgling.  Cowbirds outsource their child-rearing.

The host parents raise the hatchlings as their own offspring, although Cornell’s Birds of the World (BOTW) notes that cowbird nestlings typically get fed more often than the hosts’ offspring. They give no explanation for why, but I suppose that cowbirds have evolved to be aggressive clamorers for food.

Are you wondering about my word choice? ‘Host’ seems an overly kind way to describe birds whose nests have been usurped by cowbirds. But it’s the term BOTW uses. Here’s their explanation:

“Over 220 hosts species have been reported as being parasitized (= cowbird “victims”); 144 species have actually reared cowbird young (= cowbird hosts).”

But the news isn’t all bad for the Baltimore Orioles. BOTW explains that cowbird parasitism often fails because the orioles commonly eject cowbird eggs. They stab the eggs and fly them out whole or in pieces. So, orioles are ‘victims’ more often than ‘hosts.’ When orioles do become hosts, however, BOTW reports that the nestlings often die or are severely malnourished.

So, back to my oriole neighbors. Was the cowbird I filmed laying an egg or just casing the joint? I can’t see inside the nest, so I’ll have the answer only if I’m able to see the eventual nestlings and identify them. My camera angle may not allow that… we shall see.

I titled this post ‘High Crimes,’ using the plural because a possible case of brood parasitism isn’t the only treetop caper I’ve witnessed. Take a look:

According to BOTW, Baltimore Orioles are known to pilfer materials from other nests. There are quite a few orioles flying around our woods, so… what do you think? Did the suspect fly off with the goods?

Image Credits: Kurt Bauschardt.

4 thoughts on “High Crimes”

  1. I am relieved to hear that cowbird nestlings don’t harm oriole nestlings. With some other species, they not only grab most of the food but sometimes even shove the host nestlings out of the nest. I have to laugh when I see some of your videos. I now have more things to worry about! I dearly love orioles and know they will survive. My brother in law found 3 duck eggs on his boat cover the other day. Obviously, ducks are not as particular about their nests as orioles. LOL

    1. Well, Cornell does say that some of the oriole nestlings die, presumably from lack of food. But their overall assessment was that while oriole nests are appealing to cowbirds (shape and size), the orioles do a pretty good job of ejecting the cowbird eggs. Huzzah for the orioles!

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